As the name suggests, material handling machines such as cranes (as well as excavators, loaders and shovels) are used to move material and objects from one place to another. Both the cranes themselves and the items handled by them can take any of a wide variety of forms. For example, material handled by such machines may include excavated soil, rock and (to cite a more unusual example) the water level control gates of a waterway dam.
Cranes are made in a variety of types including overhead travelling cranes (OTC) which run on elevated rails. Another type, described in connection with the invention, is a gantry crane. A gantry crane is a "stiff-legged" structure, the shape of which resembles an inverted "U." Such a crane has a generally horizontal frame arrangement supported at an elevation by side supports or "legs." The lower ends of the legs ride on rigid flanged wheels rolling atop spaced-apart railroad-like rails.
A gantry crane, like an OTC, has a movable trolley at the top of the inverted "U" and mounted on the trolley is a load-lifting hoist or, perhaps, two or more hoists. The wheels and rails supporting the lower ends of the crane legs are below the trolley and hoist apparatus, e.g., are at ground level or, as described below, are atop a water way dam. Such cranes are capable of picking up a load, e.g., a gate of a water way dam, raising it to an elevation and transporting the load to another location.
Since the invention is described in connection with a particular type of gantry crane used to place and remove gates in a large water way dam, the following discussion will be helpful in appreciating further details of such a gate-handling crane. Following this discussion, an explanation is provided as to some of the details of a water way dam and of the gates used therein.
Regarding a gate-handling crane, the rails supporting the crane legs are atop the dam and run generally parallel to the long axis of such dam. The crane has a powered bridge apparatus which spans and rides on rails atop the elevated horizontal portion of the crane. These bridge rails are oriented generally normal to the main crane rails; that is, the bridge rails are generally parallel to the direction of water flow. Thus, the bridge can be moved in an "upstream"or "downstream" direction.
Atop the bridge is a pair of trolleys riding on bridge- mounted rails. These trolley rails, like the main rails, run generally parallel to the long axis of the dam. While the trolleys are mechanically connected together so that they move in unison, the length of the connection can be readily changed.
Each trolley has a hoist apparatus comprising a cylinder-shaped, motor-driven hoist drum with many "wraps" of rope laid in a spiral groove formed in the drum. (Parenthetically, in practice, the "rope" used with such gantry cranes is braided wire and is referred to in the trade as wire rope.) In this specification, the strand-like component wound on the drum and used to raise and lower the block is referred to as "rope." The strand-like component wound on a separate cable reel and used to manipulate a link described below is referred to as a "link line" or simply a "line."
Suspended from the rope directly below the trolley is a lifting block having at least one (and usually several) pulley-like sheaves which guide the rope as the block is lifted and lowered by the driven, rotating drum. So that a load can be manipulated by the crane, the lifting block includes a pivot-mounted link having a pair of spaced arms and a link bar extending between the lower ends of such arms. The way in which the link attaches to or is detached from a dam gate is explained below.
An exemplary water way dam has a length of several hundred feet (motor vehicles are routinely driven atop it) and a height in the range of 60 to 100 feet. Formed in the dam are several vertical slot-like openings, each of which receives one or more gates for controlling the water flow rate. A gate is lowered into an opening to retard water flow and removed to increase such flow. In the exemplary dam, there are three gates, one atop the other, in each slot-like opening. Each gate may range in size from 15 to 30 feet in height and in excess of 10 feet in width and may weigh several dozen tons.
Near the extreme end of each gate top edge is a device such as a generally U-shaped hook for lifting and lowering the gate. In one arrangement, the open interior eye-like area of the hook faces upstream. When removing a gate, the crane is positioned over the opening and the trolleys are moved so that the lifting block of each is directly above a hook. For the reasons mentioned below, attaching the block-mounted link to the hook can be very difficult and "tricky."
One reason attachment is so difficult is that while the distance between the dam top and the hooks is known, the hooks are often under water and are not visible to the crane operator. In prior art cranes, a way the operator can judge the distance of the lifting block below the trolley is by watching tape marks on the hoist rope. Such marks are subject to being worn away and in darkness, they may be even less easily seen than in daylight.
The operator may also judge such distance (or, more accurately, judge when the link has contacted the hook) by watching when the hoist rope goes slack. But this technique may not be effective when the top edge of the gate is, say, 30 to 40 feet below the water surface. The water flow forces acting against the rope may prevent the rope from going visibly slack.
Yet another factor impairing link/hook engagement is that in a prior art crane, it is difficult to ascertain link pivot position even if the elevation of the block and link is correct for hookup. The link is pivot-mounted on the lifting block to move between an angled position (from which the link bar can approach and "swing into" the open hook interior) and a vertical position. In the vertical position, the link is either attached to the hook (if the link was at the proper hook-engaging elevation when pivoted to that vertical position) or it is not possible to attach to the hook until first moving the link to the angled, approach position.
Users of the crane find it necessary to position persons above the gate to manually manipulate push-pull "tether ropes" while "feeling" for the hook with the link. Such persons are directly above water and the operation is hazardous. And with the advent of the invention, it is unnecessary.
Yet another factor impairing link/hook engagement in a prior art crane is that as the link is pivoted in the block from its vertical to its angled position, the link is prevented from returning to vertical by a ratchet-and-pawl mechanism. Once the link attains the proper angled position (and assuming the link is at the proper elevation), a pin is manipulated to release the pawl and allow the link to swing to its vertical "hookup" position. The need for pin manipulation by "remote control" with ropes adds complexity to the operation.
The invention addresses these disadvantages in a unique and imaginative way.